1. Field of the Invention
The present invention relates to a semiconductor laser and a method for fabricating the same and, more particularly, to a semiconductor laser for use in reading/writing of data from/to an-optical disk (hereinafter, referred to as “for an optical disk”) and a method for fabricating such a semiconductor laser.
2. Description of the Background Art
Conventionally, as a semiconductor laser for optical disks, an end face emitting-type semiconductor laser has been employed. Such an end face emitting-type semiconductor laser is required to operate at a high temperature of about 70° C. As a method for realizing this, a method is effective which increases the intensity of light confined within the active layer in the semiconductor laser to enhance interaction between the emitted laser light and electrons/holes.
However, if light is concentrated within the active layer in order to increase the intensity of light confined within the active layer, the emitted laser light tends to diverge in the direction perpendicular to the active layer. Therefore, with an end face emitting-type semiconductor laser which is generally utilized, the divergence of emitted laser light in the direction perpendicular to the active layer (hereinafter, referred to as “vertical radiation angle”) becomes larger than the divergence in the direction horizontal to the active layer (hereinafter, referred to as “horizontal radiation angle”). For example, a far field pattern of emitted laser light has a vertical radiation angle of 24° and a horizontal radiation angle of 8°; therefore, the emitted laser light becomes an elliptical shape.
However, in using for optical disks, the laser light is required to have a complete round shape. Therefore, there has been employed a method which shapes elliptical laser light into a complete round shape by laser light shaping means or a method which removes a portion of the perimeter of elliptical laser light to form complete round laser light. However, the former method has had a problem that the introduction of laser light shaping means increases the cost of semiconductor lasers. Also, the latter method has had a problem that the efficiency of laser light utilization is reduced, preventing generation of high power laser light.
Further, there has been a problem that when the power of emitted laser light is increased for realizing high-speed data writing into optical disks, the light emitting end faces of the semiconductor laser are degraded. In order to suppress degradation of the light emitting end faces of a semiconductor laser, there has been employed a method which forms window regions at the light emitting end faces and in the vicinity thereof in the semiconductor laser.
Conventionally, the formation of window regions has been achieved by forming portions in which quantum well layers, guide layers and barrier layers constituting the active layer in the semiconductor laser are mixed-crystallized. By forming the window regions, the energy band gaps of the quantum well layers in the active layers within the window regions are increased and, therefore, light absorption in the quantum well layers is reduced, thereby reducing degradations of the light emitting end faces of the semiconductor laser.
Also, there is another method for forming window regions. FIG. 12A is a schematic perspective view of a conventional semiconductor laser described in Mitsubishi Electronics Technologies Report February 2002 (pp. 129-132). This conventional semiconductor laser includes an n-type AlxGa1-xAs (x=xlow) lower clad layer 2, an undoped AlGaAs guide layer 3, an undoped GaAs guide layer 4, an undoped InGaAs quantum well layer 5, an undoped GaAs barrier layer 6, an undoped InGaAs quantum well layer 7, an undoped GaAs guide layer 8, an undoped AlGaAs guide layer 9, a p-type AlxGa1-xAs upper clad layer (x=xup) 10 and a p-type GaAs contact layer 11, that are sequentially deposited on an n-type GaAs substrate 1. Further, this conventional semiconductor laser has a ridge stripe portion 12 formed above n-type GaAs substrate 1, and window regions 13 formed at the light emitting end faces and in the vicinity thereof for suppressing degradations of the light emitting end faces of the semiconductor laser.
As shown in the refractive index distribution in FIG. 12B, in order to reduce the elliptical ratio (vertical radiation angle/horizontal radiation angle) of emitted laser light and, simultaneously, raise the kink level, the refractive index ncl of n-type AlGaAs lower clad layer 2 is set to a value higher than the refractive index ncu of p-type AlGaAs upper clad layer 10. Therefore, the Al composition ratios of these layers become xlow<xup.
With such a configuration, the light intensity distribution spreads toward the substrate and, therefore, becomes less influenced by the refractive index of ridge stripe portion 12, which raises the kink level. Further, since the light intensity distribution spreads toward the substrate, the elliptical ratio of emitted laser light may be reduced. Namely, when xlow=xup holds, emitted laser light has a vertical radiation angle of 31.5°, a horizontal radiation angle of 8.6° and an elliptical ratio of 3.7. On the other hand, when ncl−ncu=0.029 holds, emitted laser light has a vertical radiation angle of 23.9°, a horizontal radiation angle of 10.1° and an elliptical ratio of 2.4 (see, for example, a graph shown in pp. 130 of Mitsubishi Electronics Technologies Report February 2002).
In the case laser light shaping means is not used, as a semiconductor laser for optical disks, a semiconductor laser capable of emitting laser light with a lower elliptical ratio must be used.
However, the conventional semiconductor laser described in Mitsubishi Electronics Technologies Report February 2002 (pp. 129-132) emits laser light with an elliptical ratio of 2.4. Therefore, in the case laser light shaping means is not used, this conventional semiconductor laser has been insufficient for utilization for optical disks.
Furthermore, there has been a problem that when the elliptical ratio of laser light emitted from this conventional semiconductor laser is reduced, the ratio of light confined within the active layer -is decreased and, consequently, the threshold current is raised and the characteristic temperature (a parameter representing the ratio of increase of the threshold current relative to temperature increase) is lowered, which prevents this conventional semiconductor laser from operating at a high temperature of about 70° C.